This invention relates to a synthetic fiber that has a high strength and a high elastic modulus and to a method and an apparatus for fabricating the fiber.
For a fiber material exhibiting clear crystal dispersion, such as polyethylene, chain entanglement is minimized and the material is drawn at temperatures over a crystal dispersion temperature. By doing so, a high-strength and high-elastic-modulus fiber can be fabricated. This method is published by Lemstra et al., DSM Corp., Holland, in 1979. Such fibers are commercially available as products termed DYNEEMA (registered trademark) and SPECTRA (registered trademark).
Since the method needs to drag a chain out of a crystal without breaking the crystal, it is indispensable to draw the material at temperatures over a crystal dispersion temperature. The method is very effective obtain high chain orientation for polymers in which the clear crystal dispersion is observed, for example, polyethylene, polypropylene, and polyvinyl alcohol. However, it is not suitable for fiber materials in which the clear crystal dispersion cannot be observed, such as polyester and nylon.
In contrast to this, there is a method to obtain highly molecular chain orientation that a fiber material, after being rapidly heated, is drawn instantaneously and is cooled and solidified is reported and is generally known as a zone drawing method or a zone drawing and annealing method. This method is that, in principle, a steep temperature gradient is produced along the thread and thereby the thread is drawn at a high strain rate to drag a chain. Since the thread is deformed at a relatively an high strain rate with respect to a progressing rate of orientation-induced crystallization, an uniform deformation with a high draw ratio is possible. A fiber thus available has a high strength and a high elastic modulus by itself, and, moreover, it has become possible that a high-strength and high-elastic-modulus fiber which has required multi-step drawing is produced through a single drawing process or a smaller number of drawing processes than in a conventional way.
Since the above method does not rely on a phase transition phenomenon, such as crystal dispersion, inherent in a polymer of raw material, but only the formation of a steep temperature gradient in an axial direction of the fiber, it is not governed by the kind of polymer in principle. The method is thus applicable to many fiber materials including polyester and nylon.
In a conventional fabrication process of a synthetic polymer fiber, the heating of the thread has been controlled directly by a contact heater such as a heat pin or a heat roller, or indirectly by adjusting an ambient temperature in a heating zone through a non-contacting heater. As an example of the ambience of the heating zone, air or steam is cited.
In such a method, since a heat transition is chiefly made by a heat transfer through a fiber surface, the efficiency of the heat transition is impaired and rapid heating is difficult. Furthermore, because the heat transition is made through the fiber surface, uniform heating is generally difficult. In particular, where the ambient temperature of the heating zone is elevated for the purpose of rapid heating, a remarkable temperature difference is produced in the cross section of the fiber and makes the fiber liable to cause uneven deformation and inhomogeneous structure. Since rapid and uniform heating is impossible, the deformation rate of the fiber is highly limited, and zone drawing and annealing with a high speed are difficult.
On the other hand, a method of utilizing not the heat transfer but the heat radiation of infrared rays in order to uniformly heat a thread is disclosed in each of Japanese Patent Provisional Publication Nos. Hei 4-281011 and Hei 5-132816, which suggests that the method has a constant effect on uniform heating of the thread. However, a difference between this method and the prior art relative thereto is only that a heat transition system is changed from the heat transfer to the heat radiation. Moreover, a device identical in size with a conventional heat tube is used to heat the thread over substantially the same length as in a conventional drawing and heat-setting technique with respect to the traveling direction of the thread. Hence, the amount of thermal energy applied to the thread per unit time is almost the same as that in the prior art, and the advantages of the zone drawing and annealing method of drawing the thread for d short time after rapid heating cannot be optimized. It is for this reason that although infrared rays are collected to some extent in a plane perpendicular to the traveling direction of the thread, they are not collected in the traveling direction of the thread, and the output of an infrared source per unit length of the thread is not so high as to allow rapid heating.
A method of irradiating a thread with an infrared beam from a carbon dioxide laser to fabricate a polyester fiber that possesses a high molecular orientation and low specific gravity is disclosed in Japanese Patent Provisional Publication No. Sho 61-75811. This publication shows that, by the method, a thread is rapidly heated with infrared radiation and thus a fiber that has a high molecular orientation and low specific gravity can be made. According to the embodiment of the method, the draw ratio of the thread is limited to 1.29-4.3, and a difference in specific gravity between fibers obtained by this method and a conventional method is slight. Moreover, it is described that high-temperature heat treatment is performed under high tension after drawing, and thereby a fiber that has a high strength to some extent can be obtained.
However, the fiber after drawing, set forth in the above publication, has no high strength or high elastic modulus so enough as is required in the fiber industry. The present invention provides a synthetic fiber that has a higher strength and a higher elastic module than a conventional high-strength and high-elastic-modulus synthetic fiber and a method and an apparatus for efficiently fabricating the synthetic fiber with such a higher strength and a higher elastic modulus.
The high-strength synthetic fiber of the present invention is fabricated in such a way that a fiber, such as a polyester fiber, nylon fiber, or polyether ketone fiber, is irradiated with an infrared beam and is drawn while its thread is heated and softened at temperatures higher than a glass transition temperature.
The high-strength synthetic fiber, when fabricated by drawing a polyester fiber, has an average refractive index of 1.58-1.69, and a difference in refractive index between two principal axis caused by double refraction, hereinafter being referred to as merely xe2x80x9ca birefringencexe2x80x9d, is 0.16-0.24.
The high-strength synthetic fiber, when fabricated by drawing a polyester fiber, has a strength of 0.85-3 GPa (Giga Pascal).
The high-strength synthetic fiber is such that even when the fiber is dissolved in a solution of orthochlorophenol and the viscosity number of the solution measured on the basis of ISO 1628-5 is 0-0.65 dl/g, the strength of the fiber is still 0.85-3 GPa.
Further, the high-strength synthetic fiber, when fabricated by drawing a polyester fiber, has an initial elastic modulus of 18-40 GPa and a boil-off shrinkage of less than 4%.
The method of fabricating the high-strength synthetic fiber of the present invention is that while the material of a synthetic fiber obtained by melt spinning is made to travel at a speed of 0.1-150 meters per second, it is heated by irradiation of an infrared beam and is softened by raising fiber temperatures 20-300 K in an irradiation range so that the fiber is drawn by an external force and is wound on a reel.
The draw ratio for acquiring the high-strength fiber is 5-10 for a fiber with a birefringence of 0-0.005; 4-7 for 0.005-0.010; 3-6 for 0.010-0.020; and 1.8-5 for 0.020-0.200.
Before a process that the thread is heated and softened by the irradiation of the infrared beam, the thread may be preheated at temperatures somewhat lower than the above fiber temperatures.
The above fabricating method may also be repeated several times.
This fabricating method may be carried out so that a process for heating, softening, and drawing the thread follows a process for cooling and solidifying a thread melt-spun through a spinneret.
When the thread is heated, softened, and then drawn, vibration strain with an amplitude of 10-1000 xcexcm and a frequency of 100-100000 kHz may be applied to the thread along the axial direction of the fiber.
For the irradiation of the infrared beam, it is desirable to use a coherent light source with a laser.
The fiber fabricating apparatus of the present invention is provided with a means for continuously feeding a thread 1 at a constant feed rate v and an infrared irradiation means including a laser for irradiating the thread 1 with an infrared beam, interposed between the feeding means and a fiber winding means 11 for winding a fiber at a winding rate V higher than the constant feed rate v, in order to soften the thread 1 traveling to be fed and wound.
The infrared irradiation means can be properly used when having a lens, a mirror, a prism, and/or a wave guide to conduct infrared rays emitted from the laser to a traveling thread.
It is favorable that the lens, the mirror, the prism, and/or the wave guide are disposed so that the entire circumference of the thread is irradiated with the infrared rays.
It is also favorable that the lens, the mirror, the prism, and/or the wave guide is disposed so that the infrared rays in the traveling direction of the thread are collected in an area in which the thread is softened, while the infrared rays in a direction perpendicular to the traveling direction of the thread are collected in an area equivalent to, or somewhat larger than, the thickness of the thread.